Selective catalytic reduction (“SCR”) exhaust after-treatment systems are an important technology for reducing NOx emissions from internal combustion engines such as diesel engines. SCR systems generally include a source of reductant such as a urea solution, a pump unit for pressurizing the urea solution, a metering unit for providing a controlled amount or rate of urea solution to an SCR catalyst, and an injector which provides urea solution to a urea decomposition region of an exhaust flow path located upstream from an SCR catalyst. Many SCR systems also utilize pressurized gas to assist the flow of urea solution to the injector.
While providing important reductions in NOx emissions, SCR systems suffer from a number of shortcomings and problems. The reductant in the storage tank may be actively heated, and the feed lines from the reductant storage tank to other components may also be heated, for example by electrical heating. However, areas where the feed lines are joined to the reductant system components, such as the inlet chamber of the injector or dosing module, may not receive sufficient heat to prevent freezing or to quickly unthaw frozen reductant after engine start in a cold ambient environment. Delay time for thawing of reductant during engine warm-up can have an emissions impact on the system. Therefore, additional improvements in this technology area are needed.